i would be very careful. the documentation and application used are for the 505 and use the rom image for the 505. can anyone verify it works the same with the 600 firmware?

Yes, I can verify. It's all the same and working just fine!

1) Download this hacked FW
2) Unpack new_opt.600.img by cramfs tools
3) Prepare your font and put it to \opt\opt\sony\ebook\FONT
4) Pack it all back to new_opt.600.img file by cramfs tools
5) By using for example Microsoft Checksum Integrity Verifier get new MD5 for new_opt.600.img
6) write new MD5 to new_opt.600.md5
7) flash new fw to PRS-600